A battery pack which is detachably connected to a charger and charged by connecting thereto, includes a battery assembly including a plurality of secondary batteries serially connected; a pair of power terminals connected to a negative and a positive electrode of the battery assembly, respectively; a control circuit adapted to individually detect voltages across respective secondary batteries and output a charge control signal to the charger when the detected voltages exceed a predetermined value; a signal terminal adapted to output the charge control signal to the charger; and a temperature measurement element for detecting a temperature of the battery assembly. Further, the control circuit changes the predetermined value to be compared with the detected voltages based on the detected temperature of the temperature measurement element.